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THE FATE OF DEAD RADIO-LOUD ACTIVE GALACTIC NUCLEI: A NEW PREDICTION OF LONG-LIVED SHELL EMISSION
Ito, Hirotaka,Kino, Motoki,Kawakatu, Nozomu,Orienti, Monica IOP Publishing 2015 The Astrophysical journal Vol.806 No.2
<P>We examine the fate of a dead radio source in which jet injection from the central engine has stopped at an early stage of its evolution (t = t(j) less than or similar to 10(5) years). To this aim, we theoretically evaluate the evolution of the emission from both the lobe and the shell, which are composed of shocked jet matter and a shocked ambient medium (i.e., shell), respectively. Based on a simple dynamical model of expanding lobe and shell, we clarify how the broadband spectrum of each component evolves before and after the cessation of the jet activity. It is shown that the spectrum is strongly dominated by the lobe emission while the jet is active (t <= t(j)). On the other hand, once the jet activity has ceased (t> t(j)), the lobe emission fades out rapidly, since fresh electrons are no longer supplied from the jet. Meanwhile, shell emission only shows a gradual decrease, since fresh electrons are continuously supplied from the bow shock that is propagating into the ambient medium. As a result, overall emission from the shell overwhelms that from the lobe at a wide range of frequencies from radio up to gamma-ray soon after the jet activity has ceased. Our result predicts a new class of dead radio sources that are dominated by shell emission. We suggest that the emission from the shell can be probed in particular at radio wavelengths with the Square Kilometer Array phase 1.</P>
ACCRETION PROPERTIES OF HIGH- AND LOW-EXCITATION YOUNG RADIO GALAXIES
Son, Donghoon,Woo, Jong-Hak,Kim, Sang Chul,Fu, Hai,Kawakatu, Nozomu,Bennert, Vardha N.,Nagao, Tohru,Park, Daeseong IOP Publishing 2012 The Astrophysical journal Vol.757 No.2
<P>Young radio galaxies (YRGs) provide an ideal laboratory to explore the connection between the accretion disk and radio jet thanks to their recent jet formation. We investigate the relationship between the emission-line properties, the black hole accretion rate, and the radio properties using a sample of 34 low-redshift (z < 0.4) YRGs. We classify YRGs as high-excitation galaxies (HEGs) and low-excitation galaxies (LEGs) based on the flux ratio of high-ionization to low-ionization emission lines. Using the H alpha luminosities as a proxy of accretion rate, we find that HEGs in YRGs have similar to 1 dex higher Eddington ratios than LEGs in YRGs, suggesting that HEGs have a higher mass accretion rate or higher radiative efficiency than LEGs. In agreement with previous studies, we find that the luminosities of emission lines, in particular H alpha, are correlated with radio core luminosity, suggesting that accretion and young radio activities are fundamentally connected.</P>